1. Field of the Invention
The present invention relates to a solid-state imaging device of converting light into an electric charge for detection and, particularly, to a solid-state imaging device with improved charge detection efficiency.
2. Description of Related Art
FIG. 1 shows the configuration of a typical CCD image sensor. As shown in FIG. 1, a typical CCD image sensor 10 includes a photodiode line 11, a register 13, a transfer gate 12, and a charge detector 14. The photodiode line 11 is formed of a line of photodiodes that convert incident light into a signal electron by photoelectric conversion to accumulate electric charges. The register 13 transfers a signal charge. The transfer gate 12 transfers a signal charge from the photodiode to the register 13. The charge detector 14 converts a charge signal into a voltage signal.
In the CCD image sensor 10 having such a configuration, the electric charge which is photoelectrically converted and accumulated in each photodiode is transferred through the transfer gate 12 to the register 13. In the register 13, the electric charge is sequentially transferred by two-phase driving to the charge detector 14.
FIG. 19 shows a charge detector that is typically used in the above CCD image sensor, which is referred to hereinafter as the related art 1. The charge detector 104 includes two output gates 201 and 202 which are connected to a final-stage transfer gate (register gate) 209 and supplied with a constant voltage, a reset gate 203, and a reset drain 204 which is supplied with a constant voltage. The charge detector 104 converts an electric charge into a voltage and then transfers the voltage signal through a contact 105 to an amplifier 16 shown in FIG. 1. The amplifier 16 amplifies the voltage signal as needed and outputs the amplified signal.
The electric charge in the charge detector 104 is abandoned to the reset drain 204 to which a constant voltage is applied when the reset gate 203 is set to High, thereby entering the reference output mode in which the output signal is 0.
The CCD image sensor can convert from a smaller amount of electric charges into a larger amount of voltages as the detection efficiency is higher. The detection efficiency is a coefficient for converting an electric charge into a voltage. The high detection efficiency enables high-speed operation by suppressing the gain of the amplifier in the subsequent stage. Further, an increase in the signal gives a higher signal-to-noise (S/N) ratio.
The detection efficiency is represented by the floating capacitance (=C) of the portion for accumulating electric charges in the charge detector and an elementary charge (=e) as follows:Detection efficiency=e/C(V/electron).Thus, the detection efficiency can be enhanced if the floating capacitance C is reduced. Because the floating capacitance C is a sum of the base capacitance and the side capacitance of the portion where electric charges are accumulated (i.e. floating diffusion: FD), the capacitance can be reduced by decreasing the area of the floating diffusion and the area of the side surfaces surrounding the portion.
FIG. 20 shows another example of a charge detector as disclosed in Japanese Unexamined Patent Application Publication No. 1-196175, for example, which is referred to herein as the related art 2. FIG. 20 corresponds to the section along XX-XX′ in FIG. 19. The charge detector includes a final-stage register gate 219, an output gate 212, a contact 215, a reset gate 213a, a reset gate 213b as a reset noise reduction gate to which a constant voltage is applied, a drain 214, and a floating diffusion 216. The reset noise reduction gate 213b which is supplied with a constant current is disposed in the previous stage of the reset gate 213a, thereby enabling the reduction of reset noise.
Further, a technique of establishing connection with a wide register or a narrow charge detector without deteriorating the charge transfer efficiency is disclosed in Japanese Unexamined Patent Application Publication No. 5-325589, which is referred to herein as the related art 3. According to this technique, the use of a narrow charge detector enables the reduction of floating capacitance to thereby enhance the detection efficiency.
Because the area of the floating diffusion 216 and the area of the side surfaces surrounding this portion in the related art 2 is the same as that in the related art 1, the floating capacitance C stays the same. The detection sensitivity thus cannot be improved.
According to the related art 3, if the width of the register is inherently narrow, it is unable to reduce the floating capacitance to enhance the detection efficiency. Further, because the area of the floating diffusion cannot be reduced drastically, the effect of enhancing the detection efficiency is small.